Tetragonal magnetic phase in Ba1-x KxFe2As2 from x-ray and neutron diffraction

Combined neutron and x-ray diffraction experiments demonstrate the formation of a low-temperature minority magnetic tetragonal phase in Ba0.76K0.24Fe2As2 in addition to the majority magnetic, orthorhombic phase. The coincident enhancement in the magnetic (12 12 1) peaks shows that this minority phase is of the same type that was observed in Ba1-xNaxFe2As2 (0.24≤x≤0.28), in which the magnetic moments reorient along the c axis. This is evidence that the tetragonal magnetic phase is a universal feature of the hole-doped iron-based superconductors. The observations suggest that in this regime the energy levels of the C2 and C4 symmetric magnetic phases are very close.